Plate heat exchangers, PHEs, typically consist of two end plates in between which a number of heat transfer plates are arranged in an aligned manner, i.e. in a stack or pack. Parallel flow channels are formed between the heat transfer plates, one channel between each pair of heat transfer plates. Two fluids of initially different temperatures can flow through every second channel for transferring heat from one fluid to the other, which fluids enter and exit the channels through inlet and outlet port holes in the heat transfer plates.
Typically, a heat transfer plate comprises two end areas and an intermediate heat transfer area. The end areas comprise the inlet and outlet port holes and a distribution area pressed with a distribution pattern of projections and depressions, such as ridges and valleys, in relation to a reference plane of the heat transfer plate. Similarly, the heat transfer area is pressed with a heat transfer pattern of projections and depressions, such as ridges and valleys, in relation to said reference plane. The ridges and valleys of the distribution and heat transfer patterns of one heat transfer plate are arranged to contact, in contact areas, an upper and a lower adjacent heat transfer plate, respectively, within their respective distribution and heat transfer areas.
The main task of the distribution area of the heat transfer plates is to spread a fluid entering the channel across a width of the heat transfer plate before the fluid reaches the heat transfer area, and to collect the fluid and guide it out of the channel after it has passed the heat transfer area. On the contrary, the main task of the heat transfer area is heat transfer. Since the distribution area and the heat transfer area have different main tasks, the distribution pattern normally differs from the heat transfer pattern. The distribution pattern is such that it offers a relatively weak flow resistance and low pressure drop which is typically associated with a more “open” distribution pattern design, such as a so-called chocolate pattern, offering relatively few, but large, contact areas between adjacent heat transfer plates. The heat transfer pattern is such that it offers a relatively strong flow resistance and high pressure drop which is typically associated with a more “dense” heat transfer pattern design, such as a so-called herringbone pattern, offering more, but smaller, contact areas between adjacent heat transfer plates.
The locations and density of the contact areas between two adjacent heat transfer plates are dependent, not only on the distance between, but also on the direction of, the ridges and the valleys of both heat transfer plates. As an example, if the two heat transfer plates contain similar but mirror inverted patterns of straight, equidistant ridges and valleys, as is illustrated in FIG. 1a where the solid lines correspond to ridges of the lower heat transfer plate and the dashed lines correspond to valleys of the upper heat transfer plate, which ridges and valleys are arranged to contact each other, then the contact areas between the heat transfer plates (cross points) will be located on imaginary equidistant straight lines (dashed-dotted) which are perpendicular to a longitudinal center axis L of the heat transfer plates. On the contrary, as is illustrated in FIG. 1b, if the ridges of the lower heat transfer plate are less “steep” than the valleys of the upper heat transfer plate, the contact areas between the heat transfer plates will instead be located on imaginary equidistant straight lines which are not perpendicular to the longitudinal center axis. As another example, a smaller distance between the ridges and valleys corresponds to more contact areas. As a final example, illustrated in FIG. 1c, “steeper” ridges and valleys correspond to a larger distance between the imaginary equidistant straight lines and a smaller distance between the contact areas arranged on the same imaginary equidistant straight line.
At the transition between the distribution area and the heat transfer area, i.e. where the plate pattern changes, the strength of a pack of heat transfer plate may be somewhat reduced as compared to the strength of the rest of the plate pack due to an uneven distribution of contact areas. The more scattered the contact areas are at the transition, the worse the strength may be, since the contact areas locally may be far apart which may result in high loads in individual contact areas. Consequently, plate packs of heat transfer plates with similar but mirror inverted patterns of steep, densely arranged ridges and valleys are typically stronger at the transition than plate packs of heat transfer plates with differing patterns of less steep, less densely arranged ridges and valleys.
A plate heat exchanger may comprise one or more different types of heat transfer plates depending on its application. Typically, the difference between the heat transfer plate types lies in the design of their heat transfer areas, the rest of the heat transfer plates being essentially similar. As an example, there may be two different types of heat transfer plates, one with a “steep” heat transfer pattern, a so-called low-theta pattern, which is typically associated with a relatively low heat transfer capacity, and one with a less “steep” heat transfer pattern, a so-called high-theta pattern, which is typically associated with a relatively high heat transfer capacity. A plate pack containing only low-theta heat transfer plates may be relatively strong since it is associated with a relatively large number of contact areas arranged at the same distance from the transition between the distribution and heat transfer areas (for illustration compare with a transition between an area according to FIG. 1a and an area according to FIG. 1c). On the other hand, a plate pack containing alternately arranged high-theta and low-theta heat transfer plates may be relatively weak since it is associated with a smaller number of contact areas arranged at the same distance from the transition (for illustration compare with a transition between an area according to FIG. 1a and an area according to FIG. 1b).
A solution to the above problem is presented in applicant's own patent application WO 2014/067757, the content of which is hereby incorporated herein by reference. With reference to FIGS. 2a and 2b, which are taken from WO 2014/067757, the solution involves the provision of a transition area 2 between a distribution area 4 and a heat transfer area 6 of a heat transfer plate 8 irrespective of plate type, i.e. what a heat transfer area pattern looks like. Thereby, a transition to the distribution area will be the same irrespective of which types of heat transfer plates a plate pack contains. FIG. 2a illustrates a part of the heat transfer plate 8 as such, while FIG. 2b contains an enlargement of a portion C of the plate part of FIG. 2a and schematically illustrates the contact between the heat transfer plate 8 and an adjacent heat transfer plate.
The transition area 2 is provided with a so called herringbone pattern of ridges 10 and valleys (not illustrated). The ridges 10 are arranged to contact, in contact areas, the valleys of a similar but mirror inverted transition area of said adjacent heat transfer plate. The pattern within the transition area 2 is such that the ridges 10 and valleys are steep and densely arranged. As previously mentioned, more densely, steeper patterns may typically be associated with more closely arranged contact areas across a width of the heat transfer plate. Further, the slope of the ridges 10 and valleys within the transition area 2 is varying such that the ridges and valleys become less steep in a direction from one long side 12 to another other long side 14 of the heat transfer plate 8. In that the ridges 10 and valleys “diverge” like this, the transition area 2 contributes considerably more to an even fluid distribution across a width of the heat transfer plate than it would have done if the ridges and valleys instead had been equally steep.
The transition area 2 is bow shaped. More particularly, a borderline 16 between the transition area 2 and the distribution area 4 is, seen from the heat transfer area 6, convex and extends such that a maximum number of contact areas 18 within the distribution area 4 is arranged at the same distance from the borderline 16, and a maximum number of contact areas 20 within the transition area 2 is arranged at the same distance from the borderline 16. This makes a plate pack containing the heat transfer plate 8 relatively strong at the transition between the transition area 2 and the distribution area 4. Moreover, a borderline 22 between the transition area 2 and the heat transfer area 6 is also convex seen from the heat transfer area. It has an extension similar to a borderline (not illustrated) between two transverse sub areas of the heat transfer area to enable manufacture of heat transfer plates of different sizes containing different numbers of heat transfer sub areas by use of a modular tool. As is clear from FIG. 2b, few contact areas 24 of the heat transfer area 6 are arranged at the same distance from the borderline 22, and few contact areas 20 within the transition area 2 are arranged at the same distance from the borderline 22. This might make the plate pack relatively weak at the transition between the transition area 2 and the heat transfer area 6.